Clam AntiVirus is an anti-virus toolkit designed especially for email
scanning on mail gateways. It provides a number of utilities, including
a flexible and scalable multi-threaded daemon, a commandline scanner,
and a tool for automatic database updates. The core of the package is an
anti-virus engine available as a shared library.

John the Ripper is a fast password cracker, currently available for many flavors of Unix, Windows, DOS, BeOS, and OpenVMS. Its primary purpose is to detect weak Unix passwords. It supports several crypt(3) password hash types commonly found on Unix systems, as well as Windows LM hashes. On top of this, lots of other hashes and ciphers are added in the community-enhanced version (-jumbo), and some are added in John the Ripper Pro.

The OpenSSL Project is a collaborative effort to
develop a robust, commercial-grade, fully
featured, and Open Source toolkit implementing the
Secure Sockets Layer (SSL v2/v3) and Transport
Layer Security (TLS v1) as well as a full-strength
general-purpose cryptography library.

Lynis is an security auditing and hardening tool for Unix derivatives like Linux/BSD/Solaris. It performs an in-depth scan on the system to detect software and security issues. Besides information related to security, it will also scan for general system information, installed packages, and possible configuration mistakes. The software is aimed at assisting automated auditing, configuration management, software patch management, vulnerability detection, and malware scanning of Unix-based systems.

GnuPG (the GNU Privacy Guard or GPG) is GNU's tool for secure communication and data storage. It can be used to encrypt data and to create digital signatures. It includes an advanced key management facility and is compliant with the proposed OpenPGP Internet standard as described in RFC2440. As such, it is meant to be compatible with PGP from NAI, Inc. Because it does not use any patented algorithms, it can be used without any restrictions.

libdvdcss is a cross-platform library for transparent DVD device access with on-the-fly CSS decryption. It currently runs under Linux, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, BeOS, Win95/Win98, Win2k/WinXP, MacOS X, HP-UX, QNX, and OS/2. It is used by libdvdread and most DVD players such as VLC because of its portability and because, unlike similar libraries, it does not require your DVD drive to be region locked.

Tenable Nessus is a world-leader in active vulnerability scanners. It features high-speed discovery, configuration auditing, asset profiling, sensitive data discovery, and vulnerability analysis of your security posture. Nessus scanners may be distributed throughout an entire enterprise, inside DMZs, and across physically separate networks. It is free of charge for personal use in a non-enterprise environment.